This invention relates to a calcined, shaped alumina possessing a stabilized, high pore volume and a low surface area. More particularly, the present invention concerns a stabilized alumina shape of high total pore volume and low surface area prepared by treatment of an alumina with a stabilizing combination of silica and an inorganic fluoride, followed by calcination at high temperature.
For many catalyst applications it is important to provide a catalyst substrate or support which exhibits a high pore volume. The high porosity of the catalyst support or substrate allows the incorporation of catalytic promoters into the substrate and thus provide an active catalyst. Alumina-based catalyst supports or substrates are commonly employed since many aluminas possess the desired pore volume and pore size distribution which allow their combination with catalytic promoters. In addition, the alumina support itself does, in many instances, exhibit catalytic properties due to the active surface area of the alumina support. Narrow pore size distribution is also a preferred property of aluminas, in case of being employed as an adsorbent, selective adsorption can be achieved, and materials, having a size outside of the pore size range of the adsorbent will not be able to penetrate the pores of the adsorbent.
Low surface aluminas with controlled volume distribution in the 1 micron pore diameter range, such as prepared by the method of the invention, are especially useful for treatment of impure air. In air purification applications the alumina can be impregnated with a chemical composition, such as potassium permanganate, which removes contaminants by oxidation. Alternatively, aluminas, as produced by the instant invention, possessing low surface area and narrow pore size distribution, can be employed as supports for deodorizers or air freshening compounds, releasing air purifying compounds at a slow, controllable rate due to the narrow pore size distribution in the above-mentioned range.
In the event the catalyst is to be utilized at high temperatures or the catalyst is exposed to high temperature excursions, the stability of the alumina substrate becomes of major importance. When exposed to high temperatures for example in excess of about 1000.degree. C., alumina substrates not only alter their crystalline phases, but they also undergo other physical changes, for example the surface area, the porosity and/or the pore volume of the alumina also change, generally in an undesired direction. Activity of the catalyst can be significantly affected when the surface area decreases due to high temperature exposure. To alter the pore size distribution in a beneficial manner, i.e. to eliminate pores of very small diameter, it is customary to subject the alumina substrate to temperatures in excess of about 1500.degree. C. This however, as mentioned before, causes changes in the pore volume, it generally shrinks, which further reduces the activity of the alumina-based catalyst. Consequently, in order to provide an alumina-based catalyst support which performs in a desired manner, the substrate has to be treated to stabilize certain of its required properties.
In U.S. Pat. No. 2,630,617 alumina pebbles, used as heat exchange media at temperatures in excess of about 1650.degree. C., were stabilized against breakage and attrition by addition of small quantities of alkaline earth fluorides. The incorporation of the alkaline earth metal fluorides in the lightly calcined starting material, consisting of alpha corundum crystals, inhibited the further growth of the alpha corundum crystals when pebbles were made from the mixture and the pebbles were exposed to the above-mentioned high temperatures. No stabilization of the pore volume, the surface area or the pore size distribution were achieved. As a matter of fact, the addition of the alkaline earth metal fluoride to the alpha corundum acted, as shown in the patent, acts as a sintering agent or densifier resulting in the elimination of the pores and voids in the pebbles.
U.S. Pat. No. 4,003,851 discloses an alumina catalyst support free of thermal shrinkage and of phase change due to stabilization by exposure for a period of 24 hours to temperatures at about 980.degree. C. The process as disclosed avoids the incorporation of stabilizers in the alumina support for fear that such stabilizers may interfere with the performance of the catalyst made from the stabilized support. However, the high temperature-long term treatment disclosed in the reference admittedly results in shrinkage and consequent loss of pore volume. Also, due to the shrinkage, an unfavorable pore size distribution can be expected. These detrimental properties, derived by the high temperature treatment disclosed, counterbalance any beneficial effect that a stabilizer may have on the catalytic activity of a substrate stabilized with a chemical agent.
In U.S. Pat. No. 4,220,559 a high temperature-stable, alumina-based catalyst is described. The catalyst, used for catalyzing combustion reactions at 1000.degree.-1400.degree. C., is stabilized against phase transformation at high temperatures by incorporation of certain stabilizers, such as mixtures of the oxides of strontium or barium with silica, zirconia or stannous oxide. These stabilizers will assure that, at the high application temperatures, alpha alumina formation will be minimized and thus the active surface area of the catalyst will be retained for extended periods. The stabilizer mixtures employed in this reference reduce the surface area loss of the catalyst at the high temperatures employed in the combustion reactions. However, shrinkage of the pore volume and change in the pore size distribution are not readily achieved since the mixtures used may impart mineralizing effects which allow retention of the surface area, but not the pore volume.
It has now been discovered that an alumina adsorbent, support or substrate can be readily stabilized against high temperature deterioration, such as surface area loss and pore volume shrinkage by stabilizing the substrate with a synergistically acting mixture of silica and an inorganic fluoride.